Diesel engines normally operate at air to fuel ratios above stoichiometric. Emissions of nitrogen oxides and particulates from diesel-fueled vehicles may be significant. Emissions from diesel vehicles are subject to regulation in both the United States and Europe.
Nitrogen oxides are also present in exhaust gases from stationary sources such as power plants, industrial processes, gas turbines, etc.
One method of removing NOx in exhaust gas is to contact the exhaust stream with a reductant such as ammonia in the presence of a catalyst at elevated temperature. The catalyzed reaction of the reductant with NOx is referred to as selective catalytic reduction (SCR). Urea, ammonium hydroxide, ammonium formate, and other nitrogen-containing chemicals can also be used as a source of ammonia.
Traditional ammonia SCR catalysts are based on vanadia/titania. Imanari, et al. (U.S. Pat. No. 4,833,113), for example, describe an SCR catalyst comprising an oxide of titanium, an oxide of tungsten, and an oxide of vanadium. Vanadia/titania ammonia SCR catalysts normally operate at a temperature of approximately 250-370° C. Exhaust gas from light duty diesel vehicles is normally at a temperature of approximately 200° C. or less. Vanadia/titania SCR catalysts do not have significant activity at temperatures as low as 200° C.
Byrne (U.S. Pat. No. 4,961,917, assigned to Engelhard Corporation) disclose a method of passing ammonia, nitrogen oxides, and oxygen over iron or copper-promoted zeolite catalysts to selectively catalyze the reduction of the nitrogen oxides. The fresh copper-promoted catalyst has good activity. However, the copper catalyst deactivates significantly when aged. Although the iron catalyst is far more stable than the copper catalyst, it has maximum activity at about 350-500° C., significantly higher than the 200° C. temperatures that occur in many diesel exhaust streams.
There is a need for ammonia SCR catalysts that are stable toward aging and that are active at lower temperatures than conventional ammonia SCR catalysts.